Chris de Freitas: Human interference real threat to Pacific atolls

Atolls are formed as sea level rises around volcanic islands. Photo / NZPA

The facts around rising sea levels are widely misunderstood or misinterpreted. This has added heat to a New Zealand news story that went global: "NZ casts off first Pacific island climate change refugee".

Global warming would lead to thermal expansion of the oceans. Sea level globally has been rising at about 1.7mm a year since the Little Ice Age ended in the 19th century, but has slowed down since the high rate of rise that occurred following the last major ice age 18,000 years ago.

To the surprise of many scientists, sea level rise is barely perceptible in the Pacific. This is possibly because, at least in part, there has been no global warming over the past 17 years.

Atolls are formed as sea level rises around volcanic islands. The atolls grow as they are replenished by coral that breaks off surrounding reefs and is thrown ashore by storms. In that way atolls are self-maintaining. They have survived several periods of rapid sea level rise in the geologic past.

All remains well, provided humans don't intervene.

The digging up of an island's coral for use in construction work and the building of flush toilets that discharge the effluent into the sea where it affects coral alter nature's balance.

The environmental challenges of the Pacific atolls are diverse and sundry. They include depletion of near-shore fisheries, pollution of freshwater, soil degradation, population growth, reduction of biodiversity, damage of reef-water nursery habitats, waste management problems, and stressed natural resources related to tourism. The problems are often exacerbated by traditional approaches to land management, limited resources, small and fragile ecosystems and geographic isolation. Their relative poverty means there is a lack of adequate capacity for response.

There is some inundation evident on many atolls, which can be confused with sea level rise. It is the result of erosion, sand mining and construction projects causing an inflow of sea water. Other factors are also involved.

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Excessive use of freshwater for irrigation causes destruction of natural underground freshwater reservoirs. A consequence is seawater encroachment into vegetable growing pits, but is not the result of sea level rise.

Part of the problem is related also to the paving of the roads and land development. The effect has been to reduce infiltration of rainwater into the subsurface freshwater lens, which is the water supply source for the islanders. When this increased runoff is combined with a high tide, flooding along the coast makes it look like the sea level is rising.

Perception of trends can also be affected, as increasing population on the islands means people are now living on flood-prone land previously avoided.

Several years ago the prime minister of Tuvalu said his government was ready to sue the United States and Australia because they refused to sign the Kyoto Protocol. He claimed most of his country's atoll islands will have disappeared into the ocean within 50 years. The facts show this is not the gravest of the environmental threats the islanders face.

Chris de Freitas is an associate professor in the School of Environment at Auckland University.